Generally, in the past three types of radar have been used--pulse radar, continuous wave (CW) radar and doppler radar. The first two types are expensive and may not measure distances at close range. The third type, or doppler radar, is often used by law enforcement officers in checking motor vehicle speed. It has been generally inexpensive, reliable and accurate and has worked at close ranges. However, doppler radar has had problems measuring low velocity targets. In order for doppler radar to measure low velocities (near zero), certain circuit components including a mixer, filter amplifier and read-out circuit had to be directly coupled. However, in measuring velocities near zero, a problem existed in the amplifier and filter portion since the amplifier was required to have a direct current response and the filter was required to track down to zero frequency.
In the detector circuit of the doppler radar, a direct current was formed by diode circuitry together with an alternating current signal resulting from the reflected return doppler signal.
Due to the requirement of compensation for propagation losses, the detector signal often had to be amplified by a factor of as much as 10.sup.6. In addition to this sort of gain, to allow for wide variations in speed an amplifier was required to amplify low doppler counts, often near zero, for relatively little relative movement. An amplifier having such a magnitude of gain and operating at very low frequencies presents severe design problems. Further, the detector often had an offset on the order of one-half volt. Amplifying this extraneous voltage by a factor of 10.sup.6 could obscure the meaningful information in the doppler signal. Further, as frequencies decreased, detector noise (also known as 1/f noise) became greater. Amplification of this noise again could often mask doppler information.
Certain prior art patents relating to radar have utilized some form of phase modulation in portions of their systems. In U.S. Pat. Nos. 3,388,398; 4,019,185; 4,042,925 and 4,184,154, the transmitted radar signal was phase modulated by a pseudo-random code and the returned radar signal was cross-correlated with the pseudo-random code, apparently either in order to increase the effective detection ranges of the radar or to eliminate range ambiguities. One other patent, U.S. Pat. No. 3,750,172, used multiple frequencies rather than phase modulation in order to render the radar insensitive to targets beyond a preselected range. So far as is known, however, none of these radar systems have dealt with the problem of close ranges or near zero velocities, situations which can occur in aircraft or spacecraft.